My favorite wadding mix when I want flashing (as opposed to just "assured separation no matter what") is a mixture of AP Green Fireclay (if you can still find it) and fine sawdust 50/50. Hard to work with, but it gives nice colorations.

Most "flashing" is caused by the volitiles in the wood ash being vaporzed out by the high temperatures of the chamber firebox(es). Sodium and potassium compounds are a small component of all wood ash.... which is mostly composed of calcium compounds. At high enough temperatures this samll amount of volatile material will vaporize out of the circulating ash and the ash accumulating in the fireboxes (and on the surfaces of the logs)....... which causes the same kind of "vapor glazing" that you find in salt and soda kilns. But it is small amounts and in uneven distribution.... hence the uneven-ness of the effects. (It is like doing "residual salt" or "residula soda".....which can make a good "fake wood fire" effect.)

The longer firing cycles give these small localized deposits time to build up. Plus the longer firing is causing the clay bodies to become more vitreous as the glassy phase is speading through the crystalline body matrix (more heat work.....higher cone value), and it also allows the growth of crystalline structures like mullite (and if not controlled.... cristabolite!!!!). These "wetter" bodies are more reactive to the presence of materials nearing the surface and can "capture" a bit more of the soda / potassium compounds circulating. In addition..... these low melting surface compounds can "trap" carbon compounds.... and result in the greys, deep blues, and blacks that are often a component of some types of "flashing".

My favorite wadding mix when I want flashing (as opposed to just "assured separation no matter what") is a mixture of AP Green Fireclay (if you can still find it) and fine sawdust 50/50. Hard to work with, but it gives nice colorations.

Most "flashing" is caused by the volitiles in the wood ash being vaporzed out by the high temperatures of the chamber firebox(es). Sodium and potassium compounds are a small component of all wood ash.... which is mostly composed of calcium compounds. At high enough temperatures this samll amount of volatile material will vaporize out of the circulating ash and the ash accumulating in the fireboxes (and on the surfaces of the logs)....... which causes the same kind of "vapor glazing" that you find in salt and soda kilns. But it is small amounts and in uneven distribution.... hence the uneven-ness of the effects. (It is like doing "residual salt" or "residula soda".....which can make a good "fake wood fire" effect.)

The longer firing cycles give these small localized deposits time to build up. Plus the longer firing is causing the clay bodies to become more vitreous as the glassy phase is speading through the crystalline body matrix (more heat work.....higher cone value), and it also allows the growth of crystalline structures like mullite (and if not controlled.... cristabolite!!!!). These "wetter" bodies are more reactive to the presence of materials nearing the surface and can "capture" a bit more of the soda / potassium compounds circulating. In addition..... these low melting surface compounds can "trap" carbon compounds.... and result in the greys, deep blues, and blacks that are often a component of some types of "flashing".

Another little trick with wadding is to mix COLORANTS into it. Cobalt, copper, and so on do "jump". Depends on what you are looking for.

The best wadding I've ever used is in Japan.... called "dosembo." I have yet to figure out EXACLTY what it is. Had large irregular grains of quartz and a VERY refractory clay iwht a trace of itron content that makes our fiereclays look like earthenware. Fired at Cone 14 over 7 days.... it is about the consistency of bisque.

Been experimenting with a "synthetic mix" here to try to come close to it.... but no joy so far.